Technological advances in directional drilling within the oil industry have enabled wells to be completed with long horizontal sections extending into subsurface formations. Such long horizontal wellbores, often more than 1,000 meters long, permit fluids to be injected into or produced from a more extensive portion of a subsurface formation than would be possible using vertical wells, with commensurately greater recovery of petroleum fluids than from vertical wells. The horizontal sections of such wells are often completed with slotted steel tubulars (alternatively referred to as slotted liners) that function as screens or filters permitting flow of injected or produced fluids across the tubular wall while excluding the passage of solids.
For a slotted liner to function effectively as both a filter and a structural member in fine-grained reservoirs, and to be sufficiently rugged to endure installation handling loads, the slotted liner design is driven by three somewhat competing needs. To ensure adequate solid particle exclusion, the slot width must be on the order of the smaller sand grain sizes expected to be encountered in the formation. This is generally true even where fluids are injected out of the liner into the formation, because the effective radial stress in the sand tends to force sand grains into the well bore, even though fluids are flowing out. For reservoirs comprising very fine-grained material, slots narrower than 0.15 mm in width may be required. However, small slot widths tend to increase flow loss; therefore, a larger number of slots are needed per unit of contacted reservoir area to maintain flow capacity, while the liner must accommodate the larger number of slots without unacceptable loss of structural capacity.
The petroleum industry also recognises advantages, for production applications in particular, of slots that have a “keystone” shape in cross-section; i.e., with the flow channel through the wall of the tubular liner diverging (widening) from the external entry point to the internal exit point. This geometry reduces the tendency for sand grains to lodge or bridge in the slot, which could cause the slot to plug and restrict flow.
The required or desired width of the slots in a slotted tubular liner is commonly less than the slot width that can be formed using conventional rotary saw blades or other slot-forming technologies. Therefore, it is commonly necessary or desirable to narrow the width of the slots in slotted liners after initial formation of the slots. It is known to do this by applying pressure at or along the edges of the slots to plastically deform and displace material adjacent to the slot edges to narrow the slot width. The term “seaming”, as used in this patent document, is to be understood as denoting or referring to the process or method of narrowing the width of slots in a slotted tubular liner by this means (i.e., application of pressure to induce plastic deformation resulting in reduction of the slot width). Similarly, the terms “seamer” and “seamer head”, as used in this patent document, refer to apparatus used for purposes of seaming.
U.S. Pat. No. 6,898,957 (Slack), which is incorporated herein by reference in its entirety, teaches methods and apparatus for seaming slotted tubular liners. In accordance with certain embodiments taught by U.S. Pat. No. 6,898,957, these methods and apparatus provide at least one rigid contoured forming tool with means for applying a concentrated and largely radial load against the inside or outside cylindrical surface of a slotted metal tubular liner. The radial load thus applied at a given location on the contacted surface creates a localized zone of concentrated stress within the tubular material, which stress is sufficient to cause a significant zone of plastic deformation when the contact location is near the edge of a slot. Means are also provided for simultaneously displacing the forming tool or tools with respect to the tubular along path lines creating a typically helical sweep pattern over the cylindrical surface of the tubular. The sweep pattern is configured such that the extended zone of plastic deformation created as the forming tool passes each point on the path line covers an area sufficient to intersect the edges of all slots intended to be narrowed in width.
In accordance with methods taught in U.S. Pat. No. 6,898,957, the paths followed by the displacement of the forming tool or tools, as they follow the sweep pattern, traverse the edges of the slots a sufficient number of times and at sufficiently close intervals while maintaining sufficient contact force to plastically form the edges of all slots intersected along the slots' full lengths. The plastic deformation thus caused at the edges of the slots tends to narrow the width between opposing longitudinal edges of the slots in the contacted surface of the slotted metal tubular. Otherwise stated, the area affected by the extended zone of localized plastic flow, as the forming tool(s) move over the inside or outside surface of the slotted tubular liner, is sufficient to more than completely cover the edges of all slots to be narrowed by plastic deformation. The area swept by the forming tools need not be continuous over the entire surface of the slotted tubular liner, but optimally will include the area of influence from path lines occurring at at least two separate locations for each slot narrowed.
The steps in these methods firstly include providing a slotted tubular liner in which the slots:                extend through the tubular wall;        have longitudinal peripheral edges;        are preferably of approximately equal length;        typically have parallel slot walls (such as will result from cutting slots with a rotating saw blade); and        are preferably arranged in rows of circumferentially-distributed slots, with adjacent rows of slots being separated by unslotted intervals or rings;effectively forming a structure in which the material segments between slots act as short longitudinal beams spanning between unslotted intervals. Sub-lengths of the tubular liner having groups of one or more rows of slots are referred to as slotted intervals.        
These methods also call for the steps of providing at least one and preferably multiple contoured rigid forming tools, preferably in the form of contoured rollers, and applying pressure to a local area on the exterior surface of the tubular by means of the rigid contoured forming tools, beginning at one end of a slotted interval. At the same time, the forming tools are moved over the surface of the tubular in a tight and preferably helical sweep pattern, progressing along the length of the tubular so as to cover each slotted interval in turn. The contoured forming tool shape, the radial load exerted by the forming tools against the tubular surface, the pitch of the helical path, and the number of passes of the forming tools (i.e., the number of times the above-described operation is repeated) are all adjusted so as to result in sufficient deformation of the edges of the slots along their length to uniformly narrow each slot to a desired width.
The methods and apparatus taught in U.S. Pat. No. 6,898,957 can also be used to narrow the width of slots in a slotted tubular as measured at the interior surface of the tubular. This is achieved by using steps substantially as described above for narrowing slots at the exterior surface, except that the rigid forming tools are configured to apply pressure to the interior surface of the slotted tubular. This causes the width of each slot to be narrowed along its interior edges creating an inverse keystone flow-channel shape, which shape is desirable for injection applications (i.e., where a fluid is being injected outward from the tubular into a surrounding subsurface formation).
As outlined in U.S. Pat. No. 6,898,957, the geometry of the generally keystone channel shape created by forming the edges of slots may be further characterized in terms of the rate at which the slot width increases with depth from the contacted surface edges, i.e., its divergence rate (or the angle of the slot wall). It will be generally appreciated that slots having a lower divergence rate can be expected to plug more easily than slots with a higher divergence rate for the same reason that the keystone shape is preferred over parallel wall slots. However, if the divergence rate is very high, the formed edges will have less material supporting them and therefore will be more susceptible to material loss through erosion or corrosion. In applications where this material loss causes a significant increase in slot width, the ability to screen to the desired particle size may be compromised.
For this reason, U.S. Pat. No. 6,898,957 also teaches methods for narrowing the width of slots in slotted metal tubulars by both forming the slot edges as described above and also to control the slot divergence rate or depth to which the slot is narrowed. These objectives can be achieved by manipulating the forming tool shape according to criteria set out in U.S. Pat. No. 6,898,957.
The methods and apparatus taught by U.S. Pat. No. 6,898,957 have proven to be very effective, and large quantities of slotted tubulars are seamed every year using such methods and apparatus. However, production efficiency using methods and apparatus in accordance with U.S. Pat. No. 6,898,957 can be hampered by the common problem of tubulars having a longitudinal bend or “bowing”, typically resulting from factors such as differential cooling of longitudinal weldment areas during the manufacture of the tubulars. Such bends typically are not very dramatic, and not significant enough to cause problems with during installation or service when the tubulars are being used to make up drill strings or casing strings or as liners in horizontal wells. However, even slight longitudinal bowing can cause difficulties when present in a slotted tubular being seamed by a rotating seamer head of the type taught in U.S. Pat. No. 6,898,957.
The seamer head in U.S. Pat. No. 6,898,957 rotates about a rotational axis that is effectively fixed in space, given that the seamer head forms part of an apparatus that typically is stationary. In the ideal case, a length of slotted liner passing through the seamer head would be perfectly straight, such that its centroidal axis (i.e., centerline) would coincide with the rotational axis of the seamer head as it passes through the seamer head. In that idealized scenario, the pressures or forces exerted against the surface of the slotted tubular by all of the forming tools of the seamer head would be substantially uniform, thus promoting predictably uniform narrowing of the slots in the tubular.
However, if the centerline of the slotted liner deviates from concentricity with the rotational axis of the seamer due to an inherent longitudinal bend in the tubular, the pressures and forces exerted by the forming tools will vary, thus resulting in undesirable variations in slot width after seaming, or else entailing additional and intermittent steps to adjust the seaming equipment, or to adjust the means for supporting the non-rotating liner as it passes through the seamer (or, in some embodiments, as the seamer moves over the liner), such that the liner centerline is kept generally coincident with the rotational axis of the seamer head to facilitate acceptable quality control with respect to seamed slot width.
Although such adjustment steps may be helpful to address longitudinal bends in slotted liners that need to be run through a rotating seamer head, they decrease seaming efficiency and increase the cost of producing accurately-seamed slotted liners. Restricting seaming operations to slotted tubular liners having perfectly straight centroidal axes would be impractical and unrealistic. For these reasons, there is a need for improvements to seaming methods and apparatus that will allow longitudinally-bowed slotted liners to be seamed as effectively and efficiently as unbowed liners.